1. Field
The subject matter disclosed herein relates to determining a location based upon signals received from geo-location satellites.
2. Information
A satellite positioning system (SPS) typically comprises a system of earth orbiting satellites enabling entities to determine their location on the earth based, at least in part, on signals received from the satellites. Such an SPS satellite typically transmits a signal marked with a repeating pseudo-random noise (PN) code of a set number of chips. For example, a satellite in a constellation of a Global Navigation Satellite System (GNSS) such as GPS or Galileo may transmit a signal marked with a PN code that is distinguishable from PN codes transmitted by other satellites in the constellation.
To estimate a location at a receiver, a navigation system may determine pseudorange measurements to satellites “in view” of the receiver using well known techniques based, at least in part, on detections of PN codes in signals received from the satellites. Such a pseudorange to a satellite may be determined based, at least in part, on a code phase detected in a received signal marked with a PN code associated with the satellite during a process of acquiring the received signal at a receiver. To acquire the received signal, a navigation system typically correlates the received signal with a locally generated PN code associated with a satellite. For example, such a navigation system typically correlates such a received signal with multiple code and/or time shifted versions of such a locally generated PN code. Detection of a particular time and/or code shifted version yielding a correlation result with the highest signal power may indicate a code phase associated with the acquired signal for use in measuring pseudorange as discussed above.
To detect code phase in a signal received from a satellite in a GNSS, a navigation system may correlate a signal received from a satellite with multiple code and/or time shifted versions of locally generated PN code sequence associated with “code phase hypotheses” spanning an entire period of a periodically repeating PN code sequence. In a particular example of a GPS signal, a PN code sequence comprises 1,023 chips and repeats every millisecond. Accordingly, to detect a code phase of a signal received from a GPS satellite, a navigation system may correlate the received signal with 1,023 versions of a locally generated PN code sequence associated with the GPS satellite, phase shifted at single chip increments.
FIG. 1 illustrates an application of an SPS system, whereby a subscriber station 100 in a wireless communications system receives transmissions from satellites 102a, 102b, 102c, 102d in the line of sight to subscriber station 100, and derives time measurements from four or more of the transmissions. Subscriber station 100 may provide such measurements to position determination entity (PDE) 104, which determines the position of the station from the measurements. Alternatively, the subscriber station 100 may determine its own position from this information.
Subscriber station 100 may search for a transmission from a particular satellite by correlating the PN code for the satellite with a received signal. The received signal typically comprises a composite of transmissions from one or more satellites within a line of sight to a receiver at station 100 in the presence of noise. A correlation may be performed over a range of code phase hypotheses known as the code phase search window WCP, and over a range of Doppler frequency hypotheses known as the Doppler search window WDOPP. As pointed out above, such code phase hypotheses are typically represented as a range of PN code shifts. Also, Doppler frequency hypotheses are typically represented as Doppler frequency bins.
A correlation is typically performed over an integration time “I” which may be expressed as the product of Nc and M, where Nc is the coherent integration time, and M is number of coherent integrations which are non-coherently combined. For a particular PN code, correlation values are typically associated with corresponding PN code shifts and Doppler bins to define a two-dimensional correlation function. Peaks of the correlation function are located and compared to a predetermined noise threshold. The threshold is typically selected so that the false alarm probability, the probability of falsely detecting a satellite transmission, is at or below a predetermined value. A time measurement for the satellite is typically derived from a location of an earliest non-side lobe peak along the code phase dimension which equals or exceeds the threshold. A Doppler measurement for the subscriber station may be derived from the location of the earliest non-side lobe peak along the Doppler frequency dimension which equals or exceeds the threshold.
Correlating a received signal with multiple versions of a PN code sequence in a range of code phase hypotheses for acquisition of the received signal consumes power and processing resources. Such consumption of power and processing resources are typically critical design constraints in portable products such as mobile phones and other devices.